March 13, 2013

When an unremarkable quake becomes remarkable

Posted by Austin Elliott

>9,000 southern Californians reported feeling Monday morning's temblor

>9,000 southern Californians reported feeling Monday morning’s temblor

On Monday morning this week, a wide swath of southern California–from L.A. to Needles to Calexico–was gently jolted by a modest M4.7 earthquake from the San Jacinto fault.

On most of the planet this quake would have been unremarkable, but having shaken a population exceeding 7 million people, it earned some remarks. In fact, in the 24 hour period surrounding this earthquake there were 27 of similar or larger magnitude around the world, but this one earned the attention. (That link will probably update with time… if you need convincing you can just set up a “custom data feed” for March 11, 2013.)

The screen grab below shows all M4+ earthquakes that occurred on March 11, 2013. Most of these went unnoticed. Some were felt by many, but didn’t receive the (U.S.) coverage of the SoCal tremor.

M4+ on March 11, 2013

I love seeing the bimodal reactions of southern Californians to an earthquake of this size. No doubt the unnerving sensation of the ground suddenly shuddering beneath you is frightening to many, but there seem to be just as many who could scarcely care less.

Watch as it disrupts (or doesn’t) the coverage of the nearby Indian Wells BNP Paribas Open tennis tournament:


Southern California news media pounced on it, looking for any harrowing story of alarm or toppled tchotchkes, but came up predictably empty handed given the light and fleeting nature of the shaking. One of the big stories was that this tremor served as a further successful natural test run of California’s nacent Early Warning system, thus far only shared among a handful of scientists while it awaits further successful tests without alarming the public.

The other interesting facet of the quake was the initial determination of its location and magnitude, which was ironically marred by having a sizeable precursory foreshock, a sizable and immediate aftershock, and great instrumental sensitivity/coverage. At the outset, seismic waves from the foreshock and aftershock, which were separated from the mainshock by 16 and 51 seconds, respectively, tricked the automated system into mislocating the source and origin time of shaking. Initially the USGS identified three earthquakes with magnitudes ranging from 4.7-5.1 that occurred within two minutes of each other. Of course to most of the populace that distinction in the wavetrain would have scarcely been discernable. It was moot anyway, because as the system and seismologists further processed additional data the true sequence of events became clear. A 2.3… 16 seconds… a 4.7… 51 seconds… a 3.0. Meanwhile the shaking from each of those was rippling outward through the L.A. region and desert, overlapping with each other in their rattling. This confusion is an interesting artifact of having a great, quickly responsive seismic network… that’s not quite dense enough to pick out the details of everything it detects right away.

Southern California’s dense seismic network allows detection of minuscule earthquakes, so there is a rich foreshock and aftershock sequence evident surrounding the mainshock hypocenter. Viewing the list of foreshocks makes me intensely curious about the nucleation process of larger earthquakes.

Foreshocks and aftershocks in the 2 days before and after Monday's quake

Foreshocks and aftershocks in the 2 days before and after Monday’s quake

The L.A. Times has a good informative piece about the quake and its regional significance as expressed by USGS scientists:

Magnitude 4.7 earthquake is Monday morning’s shake-up call – L.A. Times

By the way, “Shake-Up Call” is totally in.